Optical signal processing apparatus, optical receiving apparatus, and optical relay apparatus

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 12/232,684, filed Sep. 22, 2008 now U.S. Pat. No. 8,243,363, the entire contents of which are incorporated herein by reference.

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-015593, filed on Jan. 25, 2008, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to signal light amplification utilizing a nonlinear optical effect.

2. Description of the Related Art

Communication apparatuses and communication systems utilizing optical technology have come into wide use as communication capacities and transmission distances have increased. In optical communication, transmission speed (data bit rate), total transmission capacity through a single optical fiber ((transmission rate per channel)×(number of channels)), and transmission distance are limited by waveform distortion or phase distortion of signal light, optical S/N (signal-to-noise) ratio of signal light, etc.

Waveform distortion or phase distortion of signal light is caused by chromatic dispersion (including high-order dispersion and polarization-mode dispersion), nonlinear optical effect, etc., occurring in an optical fiber forming a transmission path. To cope with waveform distortion caused by chromatic dispersion, a transmission path equipped with a normal dispersion fiber and an anomalous dispersion fiber arranged alternately, and a dispersion compensation technique using such a chromatic dispersion compensator as dispersion compensating fiber are employed.

To cope with signal light loss in an optical fiber, a technique using an optical amplifier, such as an optical fiber amplifier, is employed. Optical S/N ratio varies depending on a decrease in power due to signal light loss in an optical fiber line, amplified spontaneous emission (ASE) noises generated from the compensation of signal light loss by optical amplifier, noises generated in a receiver/transmitter, etc.

Today, a problem of grave concern is the realization of long distance transmission of signal light transmitted at a high transmission speed of 40 Gb/s, 100 Gb/s or 160 Gb/s. In high-speed transmission, however, even if a combination of a high-precision chromatic dispersion compensator and a high-quality optical amplifier is provided, reduction in the S/N ratio of signal light remains significant because of residual waveform distortion, phase distortion, and ASE noises generated from the optical amplifier. For this reason, a practical fiber transmission distance of signal light transmitted at 40 Gb/s is limited to several hundred km, and that of signal light transmitted at 160 Gb/s is limited to several km.

For the realization of long distance transmission of such high-speed signal light, it is essential to achieve an optical signal processing apparatus capable of reshaping distorted waveforms and phases, and suppressing accumulated ASE noises and phase noises. To meet this demand, optical signal processing apparatuses that controls the waveform of signal light using an optical limiter function have been disclosed such as those disclosed in, for example, Japanese Patent Application Laid-Open Publication Nos. 2000-31901 and 2000-49703.

The optical signal processing apparatus receives signal light and pump light (pulsed pump light) to a nonlinear optical medium, such as an optical fiber. The optical signal processing apparatus adjusts the relative power of signal light and pump light to saturate signal light gain resulting from a nonlinear optical effect, and thus suppresses noise in signal light having an intensity level of “1”.

The conventional optical signal processing apparatus above, however, poses a problem in that when light pulses are used as pulsed pump light, signal light cannot be amplified uniformly if the timing of signal light and the light pulses do not match. Synchronizing the signal light and the light pulses requires a clock recovery circuit, etc. In this case, different clock recovery circuits are needed according to the modulation method, bit rate, pulse width, etc., of the signal light.

Therefore, to cope with multiple types of signal light, multiple clock recovery circuits are needed, which leads to a problem of a larger and more complicated optical signal processing apparatus that invites a cost increase. To solve this problem, continuous light may be used as pump light. However, the efficiency of the occurrence of the nonlinear optical effect depends on the peak power of pump light bringing about a problem in that a large output linear optical amplifier that increases the overall power of continuous light is needed to ensure a sufficient gain by raising the efficiency of occurrence of the nonlinear optical effect.

For example, when an optical fiber is used as a nonlinear optical medium, attempts to increase the power of continuous light causes stimulated Brillion scattering in the optical fiber, thereby resulting in a part of the continuous light being reflected. Therefore, even if the power of the continuous light is increased by using a large output linear optical amplifier, sufficiently increasing the power of continuous light in the optical fiber is difficult, posing a problem in that sufficient gain cannot be ensured when continuous light is used as pump light.

When signal light is in the form of a wavelength-division multiplexed (WDM) signal, signal light in each of channels of the WDM signal arrives in random timing. For this reason, to carry out waveform reshaping of signal light for each channel, the WDM signal must be branched according to channel to reshape the waveform of each branch signal light individually. This requires multiple pump light generating circuits, clock recovery circuits, etc., each respectively corresponding to each channel, thus raising a problem of a larger and more complicated optical signal processing apparatus, inviting cost increases.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the above problems in the conventional technologies.

An optical signal processing apparatus according to one aspect of the present invention includes an input unit to which signal light is input; a wave coupling unit that couples the signal light from the input unit and pump light having a waveform different from that of the signal light; a first nonlinear optical medium that transmits light coupled by the wave coupling unit, the light being the signal light and the pump light; a dispersion medium that transmits the light that has been transmitted through the first nonlinear optical medium; and a second nonlinear optical medium that transmits the light that has been transmitted through the dispersion medium.

An optical receiving apparatus according to another aspect of the present invention includes the optical signal processing apparatus; and a receiving unit that receives the light that has been transmitted by the second nonlinear optical medium of the optical signal processing apparatus.

An optical relay apparatus according to still another aspect of the present invention includes the optical signal processing apparatus; and a receiving unit that receives the light that has been transmitted by the second nonlinear optical medium of the optical signal processing apparatus.

The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of a functional configuration of an optical signal processing apparatus according to a first embodiment;

<figref idref="DRAWINGS">FIG. 2</figref> is a block diagram of a configuration example of the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 1</figref>;

<figref idref="DRAWINGS">FIG. 3</figref> depicts allocations of the wavelengths of signal light and pulsed pump light;

<figref idref="DRAWINGS">FIG. 4</figref> is a diagram characterizing the relation between input power of the signal light and optical parametric amplification;

<figref idref="DRAWINGS">FIG. 5</figref> is a diagram characterizing the relation between the input power and the output power of the signal light;

<figref idref="DRAWINGS">FIG. 6</figref> depicts a timing shift of the signal light and the pulsed pump light;

<figref idref="DRAWINGS">FIG. 7</figref> is a block diagram of a modification of the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 2</figref>;

<figref idref="DRAWINGS">FIG. 8</figref> depicts waveform reshaping of a WDM signal by the optical signal processing apparatus;

<figref idref="DRAWINGS">FIG. 9</figref> is a block diagram of a functional configuration of an optical signal processing apparatus according to a second embodiment;

<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram of a modification of the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 9</figref>;

<figref idref="DRAWINGS">FIG. 11</figref> is a block diagram of a functional configuration of an optical signal processing apparatus according to a third embodiment;

<figref idref="DRAWINGS">FIG. 12</figref> is a flowchart of an example of control performed by the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 11</figref>;

<figref idref="DRAWINGS">FIG. 13</figref> is a block diagram of an example of an optical communication system according to a fourth embodiment; and

<figref idref="DRAWINGS">FIG. 14</figref> is a block diagram of another example of the optical communication system according to the fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, exemplary embodiments according to the present invention are explained in detail below.

<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of a functional configuration of an optical signal processing apparatus according to a first embodiment. As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the optical signal processing apparatus <b>10</b> according to the first embodiment includes a signal input unit <b>11</b>, a wave coupler <b>12</b>, a first nonlinear optical medium <b>13</b>, a dispersion medium <b>14</b>, and a second nonlinear optical medium <b>15</b>. The optical signal processing apparatus <b>10</b> amplifies signal light using a nonlinear optical effect.

The signal input unit <b>11</b> is an input unit to which signal light transmitted from another communication apparatus is input. The wavelength of signal light input to the signal input unit <b>11</b> is λ_s. A WDM signal including signal light having different wavelengths is input to the signal input unit <b>11</b>, which outputs the signal light to the wave coupler <b>12</b>.

Signal light output from the signal input unit <b>11</b> and pulsed pump light having a wavelength different from that of signal light are input to the wave coupler <b>12</b>, which is a wave coupling unit that couples input signal light and pulsed pump light. The wavelength of the pulsed pump light input to the wave coupler <b>12</b> is λ_p(not equal to λ_s). The wave coupler <b>12</b> outputs the signal light and the pulsed pump light in a coupled form to the first nonlinear optical medium <b>13</b>. The power of the signal light output from the wave coupler <b>12</b> is P_s-in, and the power of the pulsed pump light output from the wave coupler <b>12</b> is P_p.

The first nonlinear optical medium <b>13</b> transmits the signal light and the pulsed pump light output from the wave coupler <b>12</b> to the dispersion medium <b>14</b>. The power of the signal light output from the first nonlinear optical medium <b>13</b> is P_s-out, and the wavelength of the signal light output from the first nonlinear optical medium <b>13</b> is λ_s, which is the same wavelength of the signal light upon entering the first nonlinear optical medium <b>13</b>. The first nonlinear optical medium <b>13</b> is, for example, an optical fiber, a semiconductor optical amplifier having a quantum well structure, or a semiconductor optical amplifier having a quantum dot structure.

The first nonlinear optical medium <b>13</b> may be a medium that causes a second-order nonlinear optical effect such as three-wave mixing. For example, the first nonlinear optical medium <b>13</b> may be an LiNbO₃waveguide (periodically poled Lithium Niobate (PPLN)) having a pseudo phase matching structure, a GaAlAs element, or a second-order nonlinear optical crystal. In this case, three-wave mixing occurs according to the pulsed pump light traveling through the first nonlinear optical medium <b>13</b>.

The dispersion medium <b>14</b>, which is, for example, a dispersion fiber, transmits the signal light and the pulsed pump light output from the first nonlinear optical medium <b>13</b> and outputs the signal light and the pulsed pump light to the second nonlinear optical medium <b>15</b>. The second nonlinear optical medium <b>15</b> transmits and outputs the signal light and the pulsed pump light to an external destination outside the optical signal processing apparatus.

A specific example of the second nonlinear optical medium <b>15</b> is identical to that of the first nonlinear optical medium <b>13</b> and therefore, description thereof is omitted. A filter that cuts off pulsed pump light may be disposed downstream from the second nonlinear optical medium <b>15</b>. For example, a filter that cuts off light having the wavelength λ_pis disposed downstream from the second nonlinear optical medium <b>15</b>, thereby enabling extraction of only the signal light from among the signal light and the pulsed pump light having traveled through the second nonlinear optical medium <b>15</b>.

<figref idref="DRAWINGS">FIG. 2</figref> is a block diagram of a configuration example of the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 1</figref>. In <figref idref="DRAWINGS">FIG. 2</figref>, components identical to those described in <figref idref="DRAWINGS">FIG. 1</figref> are denoted by identical reference numerals, respectively and description thereof is omitted. As shown in <figref idref="DRAWINGS">FIG. 2</figref>, the optical signal processing apparatus of <figref idref="DRAWINGS">FIG. 2</figref> further includes a generating unit <b>21</b>, in addition to the components shown in <figref idref="DRAWINGS">FIG. 1</figref>. In this case, the generating unit <b>21</b> and the wave coupler <b>12</b> make up a wave coupling unit that couples the input signal light and the pulsed pump light.

The generating unit <b>21</b> generates pulsed pump light having the wavelength λp, and outputs the pulsed pump light to the wave coupler <b>12</b>. The generating unit <b>21</b> generates pulsed pump light composed of an optical pulse train having a repetitive frequency (pulse frequency) higher than the modulation rate of signal light input to the input unit <b>11</b>.

The generating unit <b>21</b> generates pulsed light having a high repetitive frequency by, for example, time division multiplexing multiple light pulses. For example, light pulse time division multiplexing is carried out by branching pulsed pump light by a branch unit, delaying part of the branched pulsed pump light by a delay circuit, and coupling every branched pulsed pump light by a wave coupler.

The generating unit <b>21</b> is, for example, a pulse laser light source. Any conventional pulse generating method is applicable in generating pulsed pump light by the generating unit <b>21</b>. Pulsed pump light is, for example, a pulse train that is generated by a semiconductor mode-locked laser or fiber mode-locked laser that is oscillated at a desired repetitive frequency.

The generating unit <b>21</b>, specifically, includes a light source and an intensity modulator, such as a LiNbO₃intensity modulator and an electronic absorption modulator. In this case, pulsed pump light is generated by the intensity modulator that modulates continuous light output from the light source with an electric signal having a desired repetitive frequency.

Pulsed pump light may be an optical comb generated by an optical phase modulator that modulates light with an electric signal having a desired repetitive frequency, or an optical pulse train generated by an optical band pass filter that extracts light in a desired wavelength band from an optical comb. Pulsed pump light may be an optical pulse train made of beating coherent light detuning at a desired repetitive frequency.

Pulsed pump light may be super continuum (SC) light that is generated by, for example, inputting light pulses having a desired repetitive frequency to a nonlinear fiber, or an optical pulse train that is generated by extracting light in a desired wavelength band from SC light through an optical band pass filter. The generating unit <b>21</b> generates pulsed pump light that has power high enough to increase the power of signal light in the first and second nonlinear optical media <b>13</b> and <b>15</b>.

In this embodiment, an optical fiber <b>22</b> and an optical fiber <b>24</b> are used as the first nonlinear optical medium <b>13</b> and the second nonlinear optical medium <b>15</b>, respectively. As the signal light is input together with the pulsed pump light to the optical fibers <b>22</b> and <b>24</b>, four-wave mixing (FWM) corresponding to accompanying pulsed pump light occurs on signal light that passes through the optical fibers <b>22</b> and <b>24</b>.

The optical fibers <b>22</b> and <b>24</b> are respectively made of the same material, and the generating unit <b>21</b> may have a configuration capable of substantially matching the wavelength λp of generated pulsed pump light to the average zero-dispersion wavelength of the optical fibers <b>22</b> and <b>24</b>. Further, the average zero-dispersion wavelength of the optical fibers <b>22</b> and <b>24</b> may be matched substantially to the wavelength λp of the generated pulsed pump light through temperature control, etc., over the optical fibers <b>22</b> and <b>24</b>. In this manner, phase matching is achieved to efficiently cause optical parametric amplification through four-wave mixing.

A method of causing the dispersion slopes of the optical fibers <b>22</b> and <b>24</b> to become linear (third-order dispersion) over a wavelength band used in the optical signal processing apparatus <b>10</b> is also effective for efficient optical parametric amplification, i.e., the quadratic dispersion of the optical fibers <b>22</b> and <b>24</b> is reduced to zero (or to a sufficiently small level) over the operating wavelength band. Phase matching may be carried out by setting the wavelength λp of pulsed pump light closer to the long wavelength side than the wavelength λs of the signal light and using nonlinear phase shifting. By these methods, optical parametric amplification through four-wave mixing can be caused more efficiently.

The optical fibers <b>22</b> and <b>24</b> must each have a sufficient length to cause the nonlinear optical effect. The length of each of the optical fibers <b>22</b> and <b>24</b> is determined so that the efficiency of occurrence of optical parametric amplification at the optical fiber <b>22</b> and at the optical fiber <b>24</b> are substantially equal. The length of each of the optical fibers <b>22</b> and <b>24</b> may be determined so that a limiter effect is optimized.

The optical fibers <b>22</b> and <b>24</b> may be, for example, a highly nonlinear optical fiber (HNLF), etc., having an enhanced nonlinear optical effect. The HNLF may be an optical fiber that is made by doping the fiber core with a germanium, bismuth-oxide fiber, chalcogenide fiber, etc., to obtain a higher nonlinear refraction factor.

The HNLF may be an optical fiber that is given a higher light power density by designing a smaller (narrower) mode field for the optical fiber, or may be a photonic crystal fiber. With the same configuration of the optical signal processing apparatus, optical Raman amplification may be adopted in place of optical parametric amplification. Whether optical parametric amplification or optical Raman amplification is to be adopted depends on the relation between the wavelength λp of the signal light and the wavelength λs of the pulsed pump light.

The dispersion medium <b>14</b> is, for example, a dispersion fiber <b>23</b> exhibiting large chromatic dispersion per unit length. For example, when signal light is in a wavelength band of 1.5 μm, the dispersion fiber <b>23</b> may be provided as a single mode fiber (SMF) having an average zero-dispersion wavelength of 1.3 μm, a dispersion compensating fiber (DCF), etc.

For instance, a case is assumed where pulsed pump light composed of a pulse train of 100 GHz is shifted in timing relative to the signal light by 5 ps, using a single mode fiber exhibiting chromatic dispersion of +20 ps/nm/km. In this case, if a wavelength difference λ_s−λ_pbetween the signal light and the pulsed pump light is assumed to be 50 nm, a single mode fiber of 5 m in length is to be used.

An optical band pass filter may be disposed on the output side of at least one of the first and second nonlinear optical media <b>13</b> and <b>15</b>. This optical band pass filter is, for example, a band pass filter that has a transmission peak near the central wavelengths of the signal light components included in WDM signal light. In the optical signal processing apparatus <b>10</b>, coupling of the signal light and the pulsed pump light generates a frequency component higher than the frequency of the signal light. This expands the spectrum of the signal light to make it larger than the spectrum at the time of input of signal light.

However, by disposing the optical band pass filter on the output end of at least one of the first and second nonlinear optical media <b>13</b> and <b>15</b>, the high-frequency component generated in the signal light can be equalized. The optical band pass filter may be, for example, an interleaver filter, a combination of fiber gratings, a cyclic filter having an resonator structure, etc. A Fabry-Perot filter may be used as the cyclic filter having the resonator structure.

<figref idref="DRAWINGS">FIG. 3</figref> depicts positions of the wavelengths of the signal light and the pulsed pump light. Optical parametric amplification at the first nonlinear optical medium <b>13</b> is described with reference to <figref idref="DRAWINGS">FIGS. 3 to 5</figref>, and the description applies further to optical parametric amplification at the second nonlinear optical medium <b>15</b>. In <figref idref="DRAWINGS">FIG. 3</figref>, the horizontal axis represents the wavelengths λ (λ_s, λ_p) of the signal light and the pulsed pump light, and the vertical axis represents the power Ps of the signal light and the power PP of the pulsed pump light. Section (a) depicts the positions of the wavelengths of the signal light and pump light input to the first nonlinear optical medium <b>13</b>.

Section (b) depicts the allocations of the wavelengths of the signal light and the pump light output from the first nonlinear optical medium <b>13</b>. Reference numeral <b>31</b> denotes the position of the wavelength of the signal light, and reference numeral <b>32</b> denotes the pulsed pump light. Although in <figref idref="DRAWINGS">FIG. 3</figref>, the wavelength λ_sof signal light input to the first nonlinear optical medium <b>13</b> is depicted to be shorter than the wavelength λ_pof pulsed pump light input to the first nonlinear medium <b>13</b>, the wavelength λ_smay be longer than the wavelength λ_p. The wavelength difference |λ_s−λ_p| between the signal light and the pulsed pump light is, for example, several nm to several ten nm.

Reference numeral <b>33</b> denotes idler light corresponding to the signal light, where the idler light is generated by four-wave mixing that occurs when pulsed pump light is input together with signal light to the first nonlinear medium <b>13</b>. The idler light is of a wavelength λ_cthat is determined by a frequency symmetrical to the frequency of signal light with respect to the frequency of pulsed pump light (ω_c=2ω_p−ω_s, where ω is frequency of waves). In four-wave mixing, part of the energy of the pulsed pump light is imparted to the signal light and to the idler light in approximately equal amounts. As a result, the signal light is amplified by optical parametric amplification, as denoted by reference numeral <b>34</b>.

A gain by optical parametric amplification increases approximately in proportional to the square of the power P_pof the pulsed pump light when the power P_pof the pulsed pump light is sufficiently higher than the power P_sof the signal light (e.g., when the power P_pis 10 times or more the power P_s). On the other hand, when the power P_pof the pulsed pump light is not sufficiently higher than the power Ps of the signal light, the consumption of the power P_pof pulsed pump light due to four-wave mixing is greater, which results in a depression that is the attenuation of the power P_pof pulsed pump light, as indicated by reference numeral <b>35</b>.

<figref idref="DRAWINGS">FIG. 4</figref> is a diagram characterizing the relation between the input power of the signal light and the optical parametric amplification. In <figref idref="DRAWINGS">FIG. 4</figref>, the horizontal axis represents the input power P_s-in(dBm) of the signal light input to the input unit <b>11</b>, and the vertical axis represents gain G_s(dB) of the signal light by optical parametric amplification at the first nonlinear optical medium <b>13</b>. The input power P_pof the pulsed pump light input to the wave coupler <b>12</b> is assumed to be constant.

When the input power P_s-inof signal light is smaller than a given threshold power P₁, the gain Gs by optical parametric amplification is constant (=G_so), as denoted by reference numeral <b>41</b>. When the input power P_s-inof signal light exceeds the threshold power P₁, a depression in the pulsed pump light occurs, which leads to a quick saturation of the gain of the signal light by optical parametric amplification. As a result, the gain G_sof the signal light by optical parametric amplification decreases, as denoted by reference numeral <b>42</b>.

While the input power P_pof pulsed pump light is described as a constant power, the relative power of the signal light and the pulsed pump light is adjusted to actually saturate optical parametric amplification. For example, the generating unit <b>21</b> outputs pulsed pump light of a power substantially identical to the power of the signal light input to the input unit <b>11</b> to saturate optical parametric amplification.

<figref idref="DRAWINGS">FIG. 5</figref> is a diagram characterizing the relation between the input power and the output power of the signal light. In <figref idref="DRAWINGS">FIG. 5</figref>, the horizontal axis represents the input power P_s-in(dBm) of the signal light input to the signal input unit <b>11</b>, and the vertical axis represents the output power P_s-out(dBm) of the signal light output from the first nonlinear optical medium <b>13</b> (or second nonlinear optical medium <b>15</b>).

A solid line represents the output power P_s-outof the signal light output from the nonlinear optical medium <b>13</b> after being subjected to optical parametric amplification in the nonlinear optical medium <b>13</b>. A dotted line represents the output power Ps-out of the signal light that results when the gain G_sby optical parametric amplification at the nonlinear optical medium <b>13</b> is zero. In this case, the input power of the pulsed pump light is assumed to be constant.

The output power P_s-outof the signal light increases with an increase in the input power P_s-inof the signal light. When the input power Ps-in of the signal light is less than the given threshold power P₁, the output power P_s-outof signal light (solid line) increases by the gain G_soas a result of optical parametric amplification, as indicated by reference numeral <b>51</b>.

When the input power P_s-inof the signal light exceeds the threshold power P₁, the gain G_sby optical parametric amplification decreases (see <figref idref="DRAWINGS">FIG. 4</figref>) in the output power P_s-outof the signal light (continuous line), which thus approaches the output power P_s-outin the case of zero gain G_sby optical parametric amplification (dotted line), as denoted by reference numeral <b>52</b>.

As a result, even when the input power P_s-inincreases, the power P_s-outdoes not further increase beyond a certain value. Hence the optical signal processing apparatus <b>10</b> operates as an optical amplifier providing a linear gain when the input power P_s-inof the signal light is equal to or less than the threshold power P₁, while operating as an optical limiter amplifier that suppresses intensity fluctuations of the output power P_s-outwhen the input power P_s-inof the signal light is greater than the threshold power P₁.

To suppress intensity fluctuations of the signal light at zero gain level, a saturable absorbing unit may be disposed upstream or downstream from at least one of the first and second nonlinear optical media <b>13</b> and <b>15</b>. The saturable absorbing unit is, for example, a semiconductor saturable absorber, semiconductor amplifier, Mach-Zehnder interferometer optical fiber switch, nonlinear optical loop mirror (NOLM) switch, etc.

<figref idref="DRAWINGS">FIG. 6</figref> depicts a timing shift of the signal light and the pulsed pump light. In <figref idref="DRAWINGS">FIG. 6</figref>, reference numerals <b>61</b> and <b>62</b> denote the waveform of the signal light and that of the pulsed pump light, respectively. When the signal light and the pump light coupled by the wave coupler <b>12</b> are input to the optical fiber <b>22</b>, only the portions of signal light that coincide with the peak portions of pulsed pump light are amplified, as indicated by reference numeral <b>63</b>.

When the signal light and the pulsed pump light output from the optical fiber <b>22</b> travel through the dispersion fiber <b>23</b>, the relative timing of the signal light and the pulsed pump light shifts. When the signal light and the pulsed pump light output from the dispersion fiber <b>23</b> are input to the optical fiber <b>24</b>, only the portions of signal light that coincide with the peak portions of the pulsed pump light are amplified.

At this time, because the relative timing of the signal light and the pulsed pump light in the optical fiber <b>22</b> is different from the relative timing of the signal light and the pulsed pump light in the optical fiber <b>24</b>, as indicated by reference numeral <b>64</b>, portions of the signal light different from the portions of the signal light amplified through the optical fiber <b>22</b> are amplified in the optical fiber <b>24</b>. As a result, as indicated by reference numeral <b>65</b>, the signal light is output from the optical fiber <b>24</b> as signal light that is uniformly amplified based on the timing the peak portion of the pulsed pump light and imparted with a temporally uniform gain.

The length of the dispersion fiber <b>23</b> is determined so that the extent of the shift of the relative timing of the signal light and the pulsed pump light is approximately half of the pulse interval of pulsed pump light. For example, when the pulsed pump light is a pulse train of approximately 100 GHz, the length of the dispersion fiber <b>23</b> is determined so that the extent of the shift of the relative timing of each pump light pulse is approximately 5 ps.

Through this configuration, the relative timing of the signal light and the pulsed pump light is shifted by a half period of the pulsed pump light in the optical fiber <b>22</b> and in the optical fiber <b>24</b>. As a result, portions of the signal light different from the portions amplified through the optical fiber <b>22</b> are primarily amplified in the optical fiber <b>24</b>.

This case is equivalent to a case where pulsed pump light having a repetitive frequency twice the frequency of this case is input to an optical fiber in a conventional configuration, which does not include the dispersion fiber <b>23</b> and the optical fiber <b>24</b>. In other words, according to the optical signal processing apparatus <b>10</b> of the first embodiment, the repetitive frequency of the pulsed pump light necessary for uniform amplification of the signal light can be reduced to half of the frequency required conventionally.

<figref idref="DRAWINGS">FIG. 7</figref> is a block diagram of a modification of the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 2</figref>. In <figref idref="DRAWINGS">FIG. 7</figref>, component identical to those described in <figref idref="DRAWINGS">FIG. 2</figref> are denoted by identical reference numerals, respectively and description thereof is omitted. As shown in <figref idref="DRAWINGS">FIG. 7</figref>, plural combinations <b>70</b> of the dispersion fibers <b>23</b> and the optical fibers <b>24</b> may be disposed downstream from the optical fiber <b>22</b> of the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 2</figref>.

With this configuration, the relative timing of the signal light and the pulsed pump light is shifted several times through the plural dispersion fibers <b>23</b> to cause the optical fibers <b>24</b> to amplify signal light at every timing shift. As a result, the signal light output from the optical fiber <b>24</b> farthest downstream is given a gain that is further temporally uniform.

If the number of the combinations <b>70</b> disposed downstream from the optical fiber <b>22</b> is N (N=2, 3, . . . ), the relative timing of the signal light and the pulsed pump light shift N times. Here, the length of each of the dispersion fibers <b>23</b> is determined so that the extent of the shift of the relative timing of the signal light and the pulsed pump light is approximately 1/(N+1) of the pulse interval of the pulsed pump light.

For example, when two combinations <b>70</b> of the dispersion fibers <b>23</b> and the optical fibers <b>24</b> are disposed downstream from the optical fiber <b>22</b>, N equals 2. In this case, the length of each of the dispersion fibers <b>23</b> is determined so that the extent of the shift of the relative timing of the signal light and the pulsed pump light is approximately ⅓ of the pulse interval of the pulsed pump light.

<figref idref="DRAWINGS">FIG. 8</figref> depicts waveform reshaping of a WDM signal by the optical signal processing apparatus. In <figref idref="DRAWINGS">FIG. 8</figref>, reference numerals <b>81</b>A to <b>81</b>E denote signal light components (wavelengths λ_Ato λ_E) included in a WDM signal input to the optical signal processing apparatus <b>10</b>. As shown in <figref idref="DRAWINGS">FIG. 8</figref>, each of signal light components included in the WDM signal input to the optical signal processing apparatus <b>10</b> arrives in random timing.

The optical signal processing apparatus <b>10</b> shifts the relative timing of each of the signal light components and the pulsed pump light through the dispersion medium <b>14</b> or the dispersion fiber <b>23</b> to enable uniform amplification of each of the signal light components regardless of the timing of arrival of each of the signal light components. This enables uniform waveform reshaping of each of the signal light components without providing plural pump light generating circuits, clock recovery circuits, etc., respectively corresponding to each channel.

According to the optical signal processing apparatus <b>10</b> of the first embodiment, signal light and pulsed pump light having wavelengths different from each other are transmitted through the dispersion medium <b>14</b> to shift the relative timing of the signal light and the pulsed pump light, thereby causing the timing of the pulsed pump light for the signal light in the first nonlinear optical medium <b>13</b> different from that for the signal light in the second nonlinear optical medium <b>15</b>. As a result, the signal light is imparted with a gain uniform in time sequence amplified uniformly.

As the optical signal processing apparatus <b>10</b> is capable of uniformly amplifying signal light merely by a configuration including the dispersion medium <b>14</b> and the second nonlinear optical medium <b>15</b>, the optical signal processing apparatus <b>10</b> has a simple configuration. For example, the optical signal processing apparatus <b>10</b> enables substantial size and cost reduction compared with an apparatus where pulsed pump light is time division multiplexed through a branch unit, delay circuit, wave coupler, etc., to increase the repetitive frequency of pulsed pump light. If time division multiplexing of signal light processing is also adopted in the optical signal processing apparatus <b>10</b>, more uniform amplification of signal light is possible.

In the first embodiment, description is made of a configuration where the generating unit <b>21</b> generates pulsed pump light composed of an optical pulse train having a repetitive frequency higher than the modulation rate of the signal light input to the input unit <b>11</b>. The configuration of the optical signal processing apparatus <b>10</b>, however, is not limited hereto. Generally, the signal light is amplified more uniformly as the repetitive frequency of the pulsed pump light increases. In addition, the optical signal processing apparatus <b>10</b> is capable of improving the efficiency of uniform amplification of the signal light in relation to the repetitive frequency of the pulsed pump light.

In the first embodiment, description is made a configuration where the length of each of the dispersion fibers <b>23</b> is determined so that the extent of the shift of the relative timing of the signal light and the pulsed pump light is approximately 1/(N+1) of the pulse interval of the pulsed pump light. The configuration of the optical signal processing apparatus <b>10</b>, however, is not limited hereto. Uniform amplification of signal light is achieved when the length of each of the dispersion fibers <b>23</b> is determined so that the extent of the shift of the relative timing of the signal light and the pulsed pump light does not coincide with the pulse interval of the pulsed pump light.

<figref idref="DRAWINGS">FIG. 9</figref> is a block diagram of a functional configuration of an optical signal processing apparatus according to a second embodiment. In <figref idref="DRAWINGS">FIG. 9</figref>, component units identical to those described in <figref idref="DRAWINGS">FIG. 2</figref> are denoted by identical reference numerals, respectively and description thereof is omitted. As shown in <figref idref="DRAWINGS">FIG. 9</figref>, the optical signal processing apparatus <b>10</b> according to the second embodiment includes a branch unit <b>91</b>, a polarization adjusting units <b>92</b>, <b>93</b>, and a wave coupler <b>94</b>, in addition to the components of the optical signal processing apparatus <b>10</b> shown in <figref idref="DRAWINGS">FIG. 2</figref>.

The branch unit <b>91</b>, polarization adjusting units <b>92</b>, <b>93</b>, and wave coupler <b>94</b> make up a polarization adjusting unit that converts pulsed pump light output from the generating unit <b>21</b> to pulsed pump light composed of two circular polarization components having polarization states orthogonal to each other. The branch unit <b>91</b> branches the pulsed pump light output from the generating unit <b>21</b>, and respectively outputs the branched pump light to the polarization adjusting units <b>92</b>, <b>93</b>.

The polarization adjusting unit <b>92</b> adjusts the polarization state of the pulsed pump light output from the branch unit <b>91</b> to be circularly polarized in a clockwise direction (circular polarization R). The polarization adjusting unit <b>92</b> outputs the pulsed pump light in the adjusted polarization state to the wave coupler <b>94</b>. The polarization adjusting unit <b>93</b> adjusts the polarization state of pulsed pump light output from the branch unit <b>91</b> to be circularly polarized in a counterclockwise direction (circular polarization L). The polarization adjusting unit <b>93</b> outputs the pulsed pump light in the adjusted polarization state to the wave coupler <b>94</b>.

The wave coupler <b>94</b> couples the pulsed pump light respectively output from the polarization adjusting units <b>92</b>, <b>93</b> and outputs the coupled pulsed pump light to the wave coupler <b>12</b>. The pulsed pump light output from the wave coupler <b>94</b> to the wave coupler <b>12</b> is, thereby, pulsed pump light composed of two circular polarization components having polarization states orthogonal to each other (circular polarization R+L). The power of each of the circular polarization components included in the pulsed pump light output to the wave coupler <b>12</b> is determined to be substantially identical.

For example, determining a branch ratio at the branch unit <b>91</b> to be 1:1 makes the power of each of the circular polarization components identical. The wave coupler <b>12</b> couples the signal light output from the signal light output unit <b>11</b> and the pulsed pump light output from the wave coupler <b>94</b>. As a result, the signal light is amplified with a constant efficiency by parametric amplification in the optical fibers <b>22</b> and <b>24</b>, regardless of the polarization state of the signal light input to the signal light output unit <b>11</b>.

<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram of a modification of the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 9</figref>. In <figref idref="DRAWINGS">FIG. 10</figref>, components identical to those described in <figref idref="DRAWINGS">FIG. 9</figref> are denoted by identical reference numerals, respectively and description thereof is omitted. As shown in <figref idref="DRAWINGS">FIG. 10</figref>, the optical signal processing apparatus <b>10</b> according to the second embodiment may include polarization adjusting units <b>101</b>, <b>102</b> in place of the polarization adjusting units <b>92</b>, <b>93</b> of the optical signal processing apparatus <b>10</b> shown in <figref idref="DRAWINGS">FIG. 9</figref>.

The branch unit <b>91</b>, the polarization adjusting unit <b>101</b>, the polarization adjusting unit <b>102</b>, and the wave coupler <b>94</b> make up a polarization adjusting unit that converts the pulsed pump light output from the generating unit <b>21</b> to pulsed pump light composed of two linear polarization components orthogonal to each other. The branch unit <b>91</b> respectively outputs the branched pulsed pump light to the polarization adjusting units <b>101</b>, <b>102</b>. The polarization adjusting unit <b>101</b> adjusts the polarization state of pulsed pump light output from the branch unit <b>91</b> to be linearly polarized (linear polarization p). The polarization adjusting unit <b>101</b> outputs the pulsed pump light in the adjusted polarization state to the wave coupler <b>94</b>.

The polarization adjusting unit <b>102</b> adjusts the polarization state of the pulsed pump light output from the branch unit <b>91</b> to be linearly polarized (linear polarization s) orthogonal to the polarization direction of the pulsed pump light output from the polarization adjusting unit <b>101</b>. The polarization adjusting unit <b>102</b> outputs the pulsed pump light in the adjusted polarization state to the wave coupler <b>94</b>, which couples the pulsed pump light output respectively from the polarization adjusting units <b>101</b>, <b>102</b>.

Pulsed pump light output from the wave coupler <b>94</b> to the wave coupler <b>12</b> is, thereby, pulsed pump light that is composed of two linear polarization components orthogonal to each other (linear polarization p+s). The power of each of the linear polarization components included in the pulsed pump light output to the wave coupler <b>12</b> is determined to be substantially identical. As a result, the signal light is amplified with a constant efficiency by parametric amplification in the optical fibers <b>22</b> and <b>24</b>, regardless of the polarization state of the signal light input to the signal light output unit <b>11</b>.

In this manner, the optical signal processing apparatus <b>10</b> according to the second embodiment achieves the same effect as that of the optical signal processing apparatus <b>10</b> according to the first embodiment. In addition, the optical signal processing apparatus <b>10</b> according to the second embodiment converts pulsed pump light output from the generating unit <b>21</b> to pulsed pump light composed of two circular polarization components having polarization states orthogonal to each other, or composed of two linear polarization components orthogonal to each other, and thus is capable of uniformly amplifying the signal light regardless of the polarization state of the signal light input to the signal input unit <b>11</b>.

While description is made of a configuration in which pulsed pump light output from the generating unit <b>21</b> is branched by the branch unit <b>91</b> and the polarization state of each branched pulsed pump light is adjusted, another configuration may be adopted in which an additional generating unit is provided and the polarization states of the pulsed pump light respectively output from the additional generating unit and the generating unit <b>21</b> are adjusted. In this case, the wavelength of the pulsed pump light respectively output from the additional generating unit and the generating unit <b>21</b> is determined to be within a wavelength band where pulsed pump light is capable of achieving a desired pump effect.

<figref idref="DRAWINGS">FIG. 11</figref> is a block diagram of a functional configuration of an optical signal processing apparatus according to a third embodiment. In <figref idref="DRAWINGS">FIG. 11</figref>, components identical to those described in <figref idref="DRAWINGS">FIG. 2</figref> are denoted by identical reference numerals, respectively and description thereof is omitted. As shown in <figref idref="DRAWINGS">FIG. 11</figref>, the optical signal processing apparatus <b>10</b> according to the third embodiment includes branch units <b>111</b>, <b>112</b>, a power monitor <b>113</b>, a comparing unit <b>114</b>, power adjusting units <b>115</b>, <b>116</b>, a branch unit <b>117</b>, a polarization monitor <b>118</b>, and a polarization adjusting unit <b>119</b>, in addition to the components of the optical signal processing apparatus <b>10</b> shown in <figref idref="DRAWINGS">FIG. 2</figref>.

The branch units <b>111</b>, <b>112</b>, and power monitor <b>113</b> make up a power monitoring unit that monitors the power of light input to the optical fiber <b>22</b> and the power of light transmitted through the optical fiber <b>24</b>. Specifically, the branch unit <b>111</b> branches part of the light output from the wave coupler <b>12</b> to the optical fiber <b>22</b> and outputs the branched light to the power monitor <b>113</b>. The branch unit <b>112</b> branches part of the light output from the optical fiber <b>24</b> to an external destination and outputs the branched light to the power monitor <b>113</b>.

The power monitor <b>113</b> monitors the power of the light output from the branch unit <b>111</b> and outputs to the comparing unit <b>114</b>, an electric signal according to the monitored power as information indicating the power of signal light before amplification thereof. The power monitor <b>113</b> monitors the power of light output from the branch unit <b>112</b> and outputs to the comparing unit <b>114</b>, an electric signal according to the monitored power as information indicating the power of signal light after amplification thereof.

The comparing unit <b>114</b> and the power adjusting units <b>115</b>, <b>116</b> make up a power control unit that controls the power of the signal light and the pulsed pump light coupled by the wave coupler <b>12</b>, based on a monitoring result obtained by the branch units <b>111</b>, <b>112</b>, and power monitor <b>113</b>. Specifically, the comparing unit <b>114</b> compares information indicating the power of signal light before amplification thereof with information indicating the power of signal light after amplification thereof, the information being output from the power monitor <b>113</b>, and calculates gain of the signal light.

The comparing unit <b>114</b> outputs information indicating the calculated gain to the power adjusting units <b>115</b>, <b>116</b>. Based on the gain information output from the comparing unit <b>114</b>, the power adjusting unit <b>115</b> adjusts the power of the signal light that is output from the input unit <b>11</b> to wave coupler <b>12</b>. Based on the gain information output from the comparing unit <b>114</b>, the power adjusting unit <b>116</b> adjusts the power of the pulsed pump light that is output from the generating unit <b>21</b> to the wave coupler <b>12</b>.

The power adjusting units <b>115</b>, <b>116</b> adjust the relative power of the signal light and the pulsed pump light input to the wave coupler <b>12</b> such that the gain indicated by information output from the comparing unit <b>114</b> is equivalent to a desired gain. For example, the power adjusting units <b>115</b>, <b>116</b> adjust the relative power of the signal light and the pulsed pump light so that a gain of the signal light is saturated, thus causing the optical signal processing apparatus <b>10</b> to operate as an optical limiter amplifier.

The branch unit <b>117</b> and the polarization monitor <b>118</b> make up a polarization monitoring unit that monitors the polarization state of the pulsed pump light input to the optical fiber <b>22</b> or to the optical fiber <b>24</b>. Specifically, the branch unit <b>117</b> branches part of the pulsed pump light included in the light output from the wave coupler <b>12</b> to the optical fiber <b>22</b> and outputs the branched light to the polarization monitor <b>118</b>. The branch unit <b>117</b> is, for example, a photocoupler that separates only the light component having a wavelength of λp from the pulsed pump light.

The polarization monitor <b>118</b> monitors the polarization state of the pulsed pump light output from the branch unit <b>117</b>. The polarization monitor <b>118</b>, for example, detects stokes parameters of the pulsed pump light output from the branch unit <b>117</b> as information indicative of the polarization state of the pulsed pump light. The polarization monitor <b>118</b> outputs to the polarization adjusting unit <b>119</b>, information indicating the monitored polarization state.

The polarization adjusting unit <b>119</b> makes up a polarization control unit that controls, based on a monitoring result from the polarization monitoring unit, the polarization state of the pulsed pump light coupled by the wave coupler <b>12</b>. Specifically, the polarization adjusting unit <b>119</b> adjusts the polarization state of the pulsed pump light output from the generating unit <b>21</b> to the wave coupler <b>12</b> so that the efficiency of the occurrence of the nonlinear optical effect, such as four-wave mixing occurring in the optical fibers <b>22</b> and <b>24</b>, is optimized.

For example, the polarization adjusting unit <b>119</b> adjusts the polarization state of the pulsed pump light so that the difference between stokes parameters output from the polarization monitor <b>118</b> and preset target stokes parameters becomes small. While description is made of a configuration in which the branch unit <b>117</b> is disposed between the wave coupler <b>12</b> and the optical fiber <b>22</b> to branch the light output from the wave coupler <b>12</b> to the optical fiber <b>22</b>, the branch unit <b>117</b> may be disposed between the optical fiber <b>22</b> and the dispersion fiber <b>23</b> or between the dispersion fiber <b>23</b> and the optical fiber <b>24</b>, as indicated by a dotted line in <figref idref="DRAWINGS">FIG. 11</figref>.

<figref idref="DRAWINGS">FIG. 12</figref> is a flowchart of an example of control performed by the optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 11</figref>. The optical signal processing apparatus shown in <figref idref="DRAWINGS">FIG. 11</figref> includes a control unit composed of a central processing unit (CPU), etc., and controls the components shown in <figref idref="DRAWINGS">FIG. 11</figref> to perform the following process. As shown in <figref idref="DRAWINGS">FIG. 12</figref>, the polarization monitor <b>118</b> monitors the polarization state of the pulsed pump light input to the optical fiber <b>22</b>, based on the pulsed pump light output from the branch unit <b>117</b> (step S<b>121</b>).

Based on the polarization state monitored at step S<b>121</b>, the polarization adjusting unit <b>119</b> adjusts the polarization state of the pulsed pump light output from the generating unit <b>21</b> to the wave coupler <b>12</b> to a desired polarization state (step S<b>122</b>). The power monitor <b>113</b> monitors, based on light output from the branch unit <b>111</b>, the power of the signal light before amplification thereof through the optical fibers <b>22</b> and <b>24</b> (step S<b>123</b>).

Based on the power monitored at step S<b>123</b>, the power adjusting unit <b>115</b> adjusts the power of the signal light output from the input unit <b>11</b> to the wave coupler <b>12</b> (step S<b>124</b>). Then, based on the light output from the branch units <b>111</b> and <b>112</b>, the power monitor <b>113</b> monitors the power of each signal light before and after amplification thereof through the optical fibers <b>22</b> and <b>24</b> (step S<b>125</b>).

Based on each power monitored at step S<b>125</b>, the comparing unit <b>114</b> calculates a gain of the signal light (step S<b>126</b>). Based on the gain calculated at step S<b>126</b>, the power adjusting unit <b>116</b> then adjusts the power of the pulsed pump light output from the generating unit <b>21</b> (step S<b>127</b>), and a series of the control process ends.

In this manner, the optical signal processing apparatus <b>10</b> according to the third embodiment offers the same effect as that according to the first embodiment. In addition, the optical signal processing apparatus <b>10</b> according to the third embodiment monitors the power of the light input to the optical fiber <b>22</b> and the power of light transmitted through the optical fiber <b>22</b>, and based on a monitoring result, controls the power of the signal light and that of the pulsed pump light, both lights being coupled by the wave coupler <b>12</b>, and is thus capable of precisely controlling gain imparted to the signal light in the optical fibers <b>22</b> and <b>24</b>.

The optical signal processing apparatus <b>10</b> according to the third embodiment further monitors the polarization state of the pulsed pump light input to the optical fiber <b>22</b> or to the optical fiber <b>24</b>, and based on a monitoring result, controls the polarization state of the pulsed pump light coupled by the wave coupler <b>12</b> and is thus capable of precisely optimizing the efficiency of the occurrence of the nonlinear optical effect in the optical fibers <b>22</b> and <b>24</b>.

Although description is made of a configuration in which the power of the signal light before and after amplification thereof through the optical fibers <b>22</b> and <b>24</b> is monitored, another configuration may be adopted in which the quality of the signal light output from the optical fiber <b>24</b> is monitored and the power of the signal light and the pulsed pump light or the polarization state of the pulsed pump light is controlled to optimize the monitored quality. The quality of signal light means various parameters, such as an optical S/N ratio, Q value, and bit error rate.

<figref idref="DRAWINGS">FIG. 13</figref> is a block diagram of an example of an optical communication system according to a fourth embodiment. As shown in <figref idref="DRAWINGS">FIG. 13</figref>, an optical communication system <b>130</b> according to the fourth embodiment includes an optical transmitting apparatus <b>131</b> and an optical receiving apparatus <b>132</b>. The optical transmitting apparatus <b>131</b> transmits to the optical receiving apparatus <b>132</b> via a transmission path, signal light modulated by intensity modulation, optical phase modulation, or optical frequency modulation. On the transmission path, optical amplifiers-repeated transmission is carried out using an optical amplifier <b>134</b> when necessary.

The optical transmitting apparatus <b>131</b> may be configured to transmit a WDM signal as signal light. The optical receiving apparatus <b>132</b> includes the optical signal processing apparatus <b>10</b> and an optical receiver <b>133</b>. The optical signal processing apparatus <b>10</b> reshapes the waveform of signal light transmitted from the optical transmitting apparatus <b>131</b> and outputs the signal light reshaped in waveform to the optical receiver <b>133</b>. The optical receiver <b>133</b> is a receiving unit that receives the signal light transmitted through the optical fiber <b>24</b> of the optical signal processing apparatus <b>10</b>.

<figref idref="DRAWINGS">FIG. 14</figref> is a block diagram of another example of the optical communication system according to the fourth embodiment. In <figref idref="DRAWINGS">FIG. 14</figref>, components identical to those described in <figref idref="DRAWINGS">FIG. 13</figref> are denoted by identical reference numerals, respectively and description thereof is omitted. As shown in <figref idref="DRAWINGS">FIG. 14</figref>, an optical communication system <b>140</b>, which is another example of the optical communication system according to the fourth embodiment, includes the optical transmitting apparatus <b>131</b>, an optical relay apparatus <b>141</b>, and an optical receiving apparatus <b>142</b>. The optical transmitting apparatus <b>131</b> transmits signal light to the optical relay apparatus <b>141</b> via a transmission path.

The optical relay apparatus <b>141</b> is a relay apparatus that relays the signal light transmitted from the optical transmitting apparatus <b>131</b> to the optical receiving apparatus <b>142</b>. The optical relay apparatus <b>141</b> includes the optical signal processing apparatus <b>10</b> and an optical transmitter <b>143</b>. The optical signal processing apparatus <b>10</b> reshapes the waveform of the signal light transmitted from the optical transmitting apparatus <b>131</b> and outputs the signal light reshaped in waveform to the optical transmitter <b>143</b>.

The optical transmitter <b>143</b> is a transmitting unit that transmits the signal light output from the optical fiber <b>24</b> of the optical signal processing apparatus <b>10</b>, to the optical receiving apparatus <b>142</b>, which receives the signal light transmitted from the optical relay apparatus <b>141</b>. The optical receiving apparatus <b>142</b> may include the optical signal processing apparatus <b>10</b> and an optical receiver <b>133</b> in a configuration similar to that of the optical receiving apparatus <b>132</b> shown in <figref idref="DRAWINGS">FIG. 13</figref>. The optical signal processing apparatus <b>10</b> described in the first to third embodiments can be applied as the optical signal processing apparatus <b>10</b> shown in <figref idref="DRAWINGS">FIGS. 13 and 14</figref>.

According to the optical communication system of the fourth embodiment, the optical signal processing apparatus <b>10</b> according to the embodiments above can be applied to the optical receiving apparatus <b>132</b> or to the optical relay apparatus <b>141</b>. The optical signal processing apparatus <b>10</b> achieves the effects described in the embodiments above and by being operated, for example, as a limiter amplifier, enables highly precise waveform reshaping. As a result, a reduction in the number of optical amplifiers <b>134</b> among communication apparatuses is enabled. In addition, the optical communication system enables suppression of ASE noises generated by the optical amplifiers <b>134</b> and as a result, the number of the optical amplifiers may be increased to extend the distance L between the communication apparatuses.

As described above, according to the optical signal processing apparatus, the optical receiving apparatus, and the optical relay apparatus of the present embodiments, signal light can be amplified uniformly using a simple configuration. Because the phase of signal light is not affected in the amplification processes by the optical signal processing apparatus <b>10</b>, the waveform reshaping method is applicable to signal light modulated by intensity modulation, optical phase modulation, optical frequency modulation, etc. For phase-modulated signal light, phase noises themselves are not suppressed; rather the optical signal processing apparatus <b>10</b> is operated as an optical limiter amplifier to reduce intensity fluctuations, thereby reducing phase noises caused by intensity fluctuations.

In optical fiber transmission, particularly, intensity noise is converted into phase noise (amplitude modulation/phase modulation (AM/PM) conversion) due to a nonlinear optical effect occurring in an optical fiber, and these AM/PM conversion noise is one of the factors that determine a transmission limit of phase-modulated signal light. Utilization of the optical signal processing apparatus <b>10</b> enables suppression of the AM/PM conversion noise.

While description is made in the embodiments of a configuration in which a WDM signal is input to the input unit <b>11</b>, the configuration of the optical signal processing apparatus <b>10</b> is not limited hereto and may be a configuration in which signal light not multiplexed in wavelength is input to the input unit <b>11</b>. In such a configuration, timing of the signal light and that of light pulses may further be matched, so that the signal light is amplified uniformly without provision of a clock recovery circuit, etc.

When signal light is amplified by optical parametric amplification in the optical fibers <b>22</b> and <b>24</b>, which are nonlinear optical media, the signal light may be subjected to the effect of cross phase modulation (XMP) whose intensity is proportional to the power of the pulsed pump light. If the signal light as a whole is subjected to XMP of the same intensity, it constitutes no problem. However, with the optical signal processing apparatus <b>10</b> where the signal light is excited by pulses with short time intervals, XPM is intense near the peak and is weak at leading edge and trailing edge of the pulses.

Further, at a portion where the slope of the nonlinear optical effect is steep, chirping proportional to the slope may occur. Even in such a case, according to the optical signal processing apparatus <b>10</b>, the effect of XPM is imparted uniformly to the signal light overall to disperse the effect of XPM by shifting the relative timing of the signal light and the pulsed pump light.

Specifically, the pulse shape and the pulse interval of pulsed pump light are adjusted so that leading edge and trailing edge of pulsed pump light substantially overlap one another in each nonlinear optical medium, thereby substantially matching the fluctuating value of XPM to the value of XPM near the peak and offsetting chirping occurring with respect to the signal light in the first and second nonlinear optical media <b>13</b> and <b>15</b>. As a result, an efficient optical limiter amplifier compensating the effect of XPM is achieved.

While description is made in the embodiments of a configuration in which pulsed pump light is used as pump light input to the wave coupler <b>12</b>, the optical signal processing apparatus <b>10</b> is applicable in a configuration other than such a configuration. For example, when the signal light is in the form of a WDM signal and continuous light is used as pump light input to the wave coupler <b>12</b>, the relative timing of each of signal light component included in the WDM signal can be shifted in the first and second nonlinear optical media <b>13</b> and <b>15</b>, thereby equalizing gain in each of the signal light components in the first and second nonlinear optical media <b>13</b> and <b>15</b>.

While description is made in the embodiments of a configuration in which the dispersion medium <b>14</b> is disposed between the first nonlinear optical medium <b>13</b> and the second nonlinear optical medium <b>15</b>, another configuration may be adopted, in which the dispersion medium <b>14</b> is not provided, rather the pulsed pump light is branched in advance according to power and into two portions, where one portion is input to the first nonlinear optical medium <b>13</b> while the other portion is temporally adjusted and then input to the second nonlinear optical medium <b>15</b>.

As a result, the timing of the pulsed pump light in the first nonlinear optical medium <b>13</b> becomes different from that in the second nonlinear optical medium <b>15</b>, thereby enabling uniform amplification of the signal light.

Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.